Studies to identify the natural substrate of the unusual mercaptan and quinone-dependent p-nitrophenylphosphatase of Clostridium sticklandii will be continued. It is of considerable interest to know whether this quinone-dependent phosphatase normally participates in a transfer process that eventually leads to ATP synthesis, whether it is involved in some energy-dependent membrane transport system of which a quinone is a component or has some other biological role in the cell. An improved and simplified isolation procedure that results in homogeneous preparations of the phosphatase in good yield renders these studies feasible.